1. Field of the Invention
The present invention pertains to a solid oxide fuel cell system, and more particularly to a solid oxide fuel cell system for generating variable electrical power in response to power demand.
2. Description of the Related Art
Solid oxide fuel cells (“SOFCs” below) are fuel cells which operate at relatively high temperatures in which, using an oxide ion-conducting solid electrolyte as electrolyte, with electrodes attached to both sides thereof, fuel gas is supplied to one side thereof and an oxidizer (air, oxygen, or the like) is supplied to the other side thereof.
In such SOFCs, steam or CO2 is produced by the reaction between oxygen ions passed through an oxide ion-conducting solid electrolyte and fuel, thereby generating electrical and thermal energy. The electrical energy is removed from the SOFC, where it is used for various electrical purposes. On the other hand, thermal energy is used to raise the temperature of the fuel, the reformer, the water, the oxidant, and the like.
Unexamined Patent Application 2010-92836 (Patent Document 1) sets forth a fuel cell device. This fuel cell device is a solid oxide fuel cell system of the type which changes generated power in response to power demand; operation is disclosed in which a fuel utilization rate is reduced more when it operates in the range of low load than when it operates in the range of high power generation load. That is, in Patent Document 1, a proportion of fuel used for power generation with respect to the entire supplied fuel is reduced when generated power is in a low state, but on the other hand fuel used to heat the fuel cell module and not used to generate electricity is not greatly reduced, and a large fraction of the fuel is used to heat the fuel cell module so that the fuel cell module is made thermally independent, and a temperature at which power generation can occur is maintained.
Specifically, when it operates in the range of low generated power, heat generated in the fuel cell unit in association with electrical generation declines. As a result, the temperature inside the fuel cell module tends to decline. Therefore, if the fuel utilization rate is maintained at a certain level even when it operates in the range of low power generation, a decline of the temperature inside the fuel cell module is induced, and it becomes difficult to maintain the temperature at which power can be generated. Therefore, fuel used to heat the fuel cell module is increased in order to sustain thermal independence, even at the sacrifice of the fuel utilization rate.
In the fuel cell device set forth in Unexamined Patent Application 2010-92836, in order to resolve these problems the fuel utilization rate is reduced when the device operates in the range of low load where electrical generation is small, thereby preventing an excessive temperature drop in the fuel cell module while stably maintaining a fixed high temperature state.
Published Unexamined Patent Application 2009-104886 (Patent Document 2), on the other hand, sets forth a method of operating a fuel cell system during an increase of load on a fuel cell system. In this operating method, when electrical generation needs to be increased in the fuel cell system, the air supply rate is first increased, and then, after the water supply rate and the fuel supply rate are increased in that order, the amount of electrical power extracted is increased. In this fuel cell system operating method, air depletion, carbon deposition, and fuel depletion are prevented from occurring by increasing the supply rates in the order described.
In addition, the operating temperature of a solid oxide fuel cell is usually high. The fuel cell cells need to be maintained at a high operating temperature when power is being generated. Therefore, it is important factors for increasing the overall energy efficiency of a fuel cell system to reduce the amount of heat dispersed into the outside atmosphere from the fuel cells and reduce the amount of fuel required to maintain the temperature of a fuel cell system. It is therefore desirable that the fuel cells be housed in a chassis with good heat insulating properties.
Published Unexamined Patent Application 2010-205670 (Patent Document 3), on the other hand, sets forth an operating method for a fuel cell system and fuel cell. In this fuel cell system, an integral value for the electrical load on the fuel cell is obtained, and the fuel utilization rate is controlled based on the obtained integral value. Control of the fuel utilization rate is performed by estimating the fuel cell temperature based on the integral value of the fuel cell electrical load. Then, the fuel utilization rate is controlled based on the estimated result. The fuel cell can therefore be operated in a thermally independent manner without the use of a temperature sensor. When the integral value of the electrical load is at or above a predetermined value, the controller corrects the fuel utilization rate to a value equal to or greater than a reference value at which the fuel cell can operate thermally independently. In this case, because of the fact that the temperature of the fuel cell has already been high, the fuel cell has surplus heat, and thermally self-sustaining operation can be maintained even if the fuel utilization rate is corrected to a value equal to or greater than the reference value at which thermal self-sustaining operation is possible. The system efficiency of the fuel cell system is thus improved.
3. Prior Art References
Patent Documents:
Patent Document 1—Published Unexamined Patent Application 2010-92836
Patent Document 2—Published Unexamined Patent Application 2009-104886
Patent Document 3—Published Unexamined Patent Application 2010-205670